23s 



THE AMERICAN NATURALIST [Vol. XLVIII 



one on each side of the abdomen of the male, near the end. 

 As the specimens, when placed on a microscope slide, 

 nearly always lie either on the dorsal or ventral side, 

 these spines are nearly always readily visible if present. 

 I nsed this criterion (mentioned in the re-description of 

 the species by Hinds, 1902) only after having taken eleven 

 pairs of this species copulating- in nature, and observing 

 in every case that the male possessed these spines, and 

 that in the female they were wanting. In other Tubu- 

 Ufera, e. g., Antliothrips uifjrr, sex was determined by the 

 longitudinal ehitinous rod in the next to the last abdomi- 

 nal segment of the female. When the specimens were too 

 opaque to observe this rod, they were cleared by boiling 



The data from these collections are given in the accom- 

 panying table, rnfortnnately the collections could not 

 all be made in one year, nor in the same locality. Those 

 made from July 1 to September 18, 1912, were made at 

 the University of Michigan Biological Station, Douglas 

 Lake, Michigan; all others were made at Ann Arbor, 

 Michigan. It is not probable that the results are greatly 

 modified by collecting in two regions within the state. 

 In this table the larvae of all species are combined, as I 

 am unable to distinguish with certainty the larva? of 

 several of the species here mentioned. 



The important facts contained in this table are, it seems 

 to me, the following : 



Euthrips tritici appeared in spring at first only in the 

 female sex. Males were first collected nearly a month 

 later, and not until about the time fairly large larvae were 

 found elsewhere. Once the males appear, though their 

 number fluctuates in the individual collections, they fur- 

 nish a fairly constant proportion of the whole number 

 (about one third). 



The males of Ant hoth rips rrrbasci appear in the earli- 

 est collection of this species, and in considerable numbers 

 throughout the season. The total proportion of males is 

 23 per cent., and the only considerable increases over 



